1. Field of the Invention
The present invention relates in general to an end mill, and more particularly to such an end mill capable of exhibiting an excellent durability even in its use for cutting a tough work material, for example, a heat resistant alloy such as kovar, inconel and stainless steel.
2. Discussion of the Related Art
There is known a square end mill including a cylindrical body made of a cemented carbide and having (a) a plurality of flutes, (b) a plurality of peripheral cutting edges which are provided by the respective flutes, and (c) a plurality of end cutting edges which are located at the axially distal end of the cylindrical body and are contiguous to the respective peripheral cutting edges. As an example of such a square end mill, JP-Y2-2557189 (publication of Japanese Utility Model issued in 1997) discloses an end mill in which one of the plurality of peripheral cutting edges has a radial rake angle smaller than that of each of the other peripheral cutting edges while one of the plurality of end cutting edges extends so as to substantially reach an axis of the cylindrical body. The end cutting edge, substantially reaching the axis, serves as a so-called “center cutting edge”, and has a shape different from that of each of the other end cutting edges as seen in a bottom view of the end mill. The center cutting edge has a negative radial rake angle, namely, is inclined such that its radially outer end is located rearward of its radially inner end as viewed in a rotating direction of the end mill, while each of the other end cutting edges has a positive radial rake angle, so that an easy breakage of the center cutting edge can be prevented without suffering from a chattering which would deteriorate the surface finish and machining accuracy.
However, even the end mill as disclosed in JP-Y2-2557189 is not capable of exhibiting a sufficiently high degree of durability when it is used for cutting a tough work material such as a heat resistant alloy (e.g., kovar, inconel and stainless steel). Since such a tough work material inherently tends to produce chips not easily broken into small pieces, a large cutting resistance acts on the end mill during the cutting of the tough work material, causing a chattering and chipping of the cutting edges. Thus, the durability of the end mill is deteriorated when used for the tough work material. For preventing the deterioration in the durability of the end mill, the cutting speed such as a feed rate has to be reduced.